A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive traces, pads and other features etched from electrically conductive sheets, such as copper sheets, laminated onto a non-conductive substrate. Multi-layered printed circuit boards are formed by stacking and laminating multiple such etched conductive sheet/non-conductive substrate. Conductors on different layers are interconnected with vias. Vias can be mechanically or laser drilled. Vias can have copper plating on a portion of the sidewalls or be plated completely.
A printed circuit board includes a plurality of stacked layers, the layers made of alternating non-conductive layers and patterned conductive layers. The non-conductive layers can be made of prepreg or base material that is part of a core structure, or simply core. Prepreg is a fibrous reinforcement material impregnated or coated with a thermosetting resin binder, and consolidated and cured to an intermediate stage semi-solid product. Prepreg is used as an adhesive layer to bond discrete layers of multilayer PCB construction, where a multilayer PCB consists of alternative layers of conductors and base materials bonded together, including at least one internal conductive layer. A base material is an organic or inorganic material used to support a pattern of conductor material. A core is a metal clad base material where the base material has integral metal conductor material on one or both sides. A laminated stack is formed by stacking multiple core structures with intervening prepreg and then laminating the stack.
In some applications, it is desirable to form a flexible portion, or section, of a printed circuit board that is bendable yet remains interconnected to other rigid sections of the printed circuit board, thereby forming a rigid-flexible printed circuit board. The flexible portion is typically not glass-reinforced and is composed of thinner layers, thus decreasing the bending stiffness and making it flexible. In still other applications, the entire printed circuit board is flexible, thereby forming a flexible printed circuit board.
Rigid-flexible printed circuit boards and flexible printed circuit boards are increasingly being used in applications, such as wearable electronics, where the flexible sections are subject to repeated bending, twisting, torquing or other such deformation relative to its static state. Such repeated deformation subjects the flexible section of the printed circuit board to mechanical stresses and strains, possibly leading to circuit failure. An example of such an application is that observed in shoes, where impact of a shoe to ground results in single or multiple strain and strain events.